Transistors are used as switches to electrically couple or decouple signals among different nodes. For example, in a mobile communication system capable of transmitting and receiving signals at various carrier frequency bands, an antenna is usually shared by various corresponding Intermediate Frequency (IF) and/or baseband circuits through one or more Radio Frequency (RF) switches. The term “RF” refers to a radio wave having a frequency ranging from about 3 kHz to 300 GHz. When two transistors sharing the same substrate are used as switches, an electrical coupling path is formed between the two transistors through the substrate.